c# - DataItem=null on binding，找不到原因？

Question

我正在尝试重现 Sheridan's answer to this question 中的建议在将 WPF 与 MVVM 模式结合使用时浏览我的 View 。不幸的是，我这样做时遇到了绑定(bind)错误。这是确切的错误:

System.Windows.Data Error: 4 : Cannot find source for binding with reference 'RelativeSource FindAncestor, AncestorType='JollyFinance.ViewModels.MainViewModel', AncestorLevel='1''. BindingExpression:Path=DataContext.DisplayTest; DataItem=null; target element is 'Button' (Name=''); target property is 'Command' (type 'ICommand')

当我查看 LoginView.xaml 中的 xaml 代码时，我注意到 Visual Studio 告诉我它无法在 MainViewModel 类型的上下文中找到 DataContext.DisplayText。我尝试删除 DataContext. 并只保留 DisplayText，但无济于事。

除非 Sheridan 的回答有误，否则我肯定遗漏了一些东西。我应该怎么做才能让它发挥作用？

主窗口.xaml:

<Window x:Class="JollyFinance.Views.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:vm="clr-namespace:JollyFinance.ViewModels"
        xmlns:views="clr-namespace:JollyFinance.Views"
        Title="JollyFinance!" Height="720" Width="1280">

    <Window.Resources>
        <!-- Different pages -->
        <DataTemplate DataType="{x:Type vm:LoginViewModel}">
            <views:LoginView/>
        </DataTemplate>
        <DataTemplate DataType="{x:Type vm:TestViewModel}">
            <views:Test/>
        </DataTemplate>
    </Window.Resources>

    <Window.DataContext>
        <vm:MainViewModel/>
    </Window.DataContext>

    <Grid>
        <ContentControl Content="{Binding CurrentViewModel}"/>
    </Grid>
</Window>

MainViewModel.cs:

public class MainViewModel : BindableObject
{
    private ViewModelNavigationBase _currentViewModel;

    public MainViewModel()
    {
        CurrentViewModel = new LoginViewModel();
    }

    public ICommand DisplayTest
    {
        get
        {
            // This is added just to see if the ICommand is actually called when I press the
            // Create New User button
            Window popup = new Window();
            popup.ShowDialog();

            // View model that doesn't contain anything for now
            return new RelayCommand(action => CurrentViewModel = new TestViewModel());
        }
    }

    public ViewModelNavigationBase CurrentViewModel
    {
        get { return _currentViewModel; }
        set
        {
            if (_currentViewModel != value)
            {
                _currentViewModel = value;
                RaisePropertyChanged("CurrentViewModel");
            }
        }
    }
}

登录 View .xaml:

<UserControl x:Class="JollyFinance.Views.LoginView"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
             xmlns:vm="clr-namespace:JollyFinance.ViewModels"
             mc:Ignorable="d" 
             d:DesignHeight="300" d:DesignWidth="300">

    <UserControl.DataContext>
        <vm:LoginViewModel/>
    </UserControl.DataContext>

    <Grid>

        <Grid.RowDefinitions>
            <RowDefinition Height="*"/>
            <RowDefinition Height="Auto"/>
            <RowDefinition Height="Auto"/>
            <RowDefinition Height="Auto"/>
            <RowDefinition Height="*"/>
        </Grid.RowDefinitions>

        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="*"/>
            <ColumnDefinition Width="Auto"/>
            <ColumnDefinition Width="Auto"/>
            <ColumnDefinition Width="Auto"/>
            <ColumnDefinition Width="Auto"/>
            <ColumnDefinition Width="*"/>
        </Grid.ColumnDefinitions>

        <TextBlock Text="Username: " Grid.Column="1" Grid.Row="1" Margin="5"/>
        <TextBox Text="{Binding Path=Username}" Grid.Column="2" Grid.Row="1" Grid.ColumnSpan="2" Margin="5"/>

        <TextBlock Text="Password: " Grid.Column="1" Grid.Row="2" Margin="5"/>
        <PasswordBox x:Name="PasswordBox" PasswordChar="*" Grid.Column="2" Grid.ColumnSpan="2" Grid.Row="2" Margin="5"/>

        <Button Content="Log In" Grid.Column="2" Grid.Row="3" Margin="5" Padding="5" Command="{Binding LoginCommand}"/>
        <Button Content="Create new user" Grid.Column="3" Grid.Row="3" Margin="5" Padding="5" 
                Command="{Binding DataContext.DisplayTest, RelativeSource={RelativeSource AncestorType={x:Type vm:MainViewModel}}, 
            Mode=OneWay}"/>

    </Grid>

</UserControl>

登录 View 模型.cs:

public class LoginViewModel : ViewModelNavigationBase
{
    public LoginViewModel()
    {
        LoginCommand = new RelayCommand(Login);
    }

    private void Login(object param)
    {
        // Just there to make sure the ICommand is actually called when I press the
        // Login button             
        Window popup = new Window();
        popup.ShowDialog();
    }

    public String Username { get; set; }

    public String Password { get; set; }

    public ICommand LoginCommand { get; set; }
}

ViewModelNavigationBase 只是一个实现了 INotifyPropertyChanged 接口(interface)的类，而 Test.xaml 和 TestViewModel.cs 只是一个用于测试目的的虚拟 View 模型/ View 。

Answer 1

在我的回答中，我声明您应该在 App.xaml 中声明您的 View 模型 DataTemplate，以便每个 View 都可以访问它们。将它们放在 MainWindow 类中是您的第一个问题。

另一个错误是您的 ICommand 的 Binding Path。如果您想从设置为 Window.DataContext 的 View 模型访问某些内容，那么您应该不使用RelativeSource Binding。试试这个:

<Button Content="Create new user" Grid.Column="3" Grid.Row="3" Margin="5" Padding="5" 
    Command="{Binding DataContext.DisplayTest}, Mode=OneWay}" />

还要记住，无论出于何种原因，您选择不让您的 MainViewModel 类扩展 ViewModelNavigationBase 类...这也可能导致你的问题。

无论如何，如果这不能解决您的问题，请告诉我。此外，如果您想在 Stack Overflow 上随时通知用户，只需在他们的名字前面放一个 @ 符号，他们就会收到通知。如果你这样做的话，你可以直接问我这个问题。